1. Field of the Invention
This invention relates to the processing of heavy crude and residual petroleum charge stocks, and in particular, relates to the visbreaking of such charge stocks containing significant quantities of metal values found to promote the formation of coke, notably, those of nickel and/or vanadium.
2. Description of the Prior Art
"Visbreaking", or viscosity breaking, is a well known petroleum refining process in which reduced crudes are pyrolyzed, or cracked, under comparatively mild conditions without significant coke production to provide products having lower viscosities and pour points thus reducing the amounts of less-viscous and more valuable blending oils required to make the residual stocks useful as fuel oils. In a typical visbreaking process, the crude or resid feed is passed through a heater and thereafter into a reaction chamber operating at from about 800.degree. to about 975.degree. F. and at about 50 to about 1000 psig. Light gas oil is injected to lower the temperature of the effluent to within about 830.degree. to about 850.degree. F. Cracked products from the reaction chamber are introduced to a flash distillation unit with the vapor overhead being separated in a fractionating column into a light distillate overhead product (i.e., gasoline) and light gas-oil bottoms, and the liquid bottoms being separated in a vacuum fractionating column into heavy gas-oil distillate and residual tar.
Heretofore, high-metals heavy charge stocks have been processed in coking or catalytic hydroprocessing operations. Visbreaking has achieved little importance with such crudes due to their tendency to produce significant quantities of coke which plugs the reactor, shortens production runs and results in unacceptably lengthy periods of down time.
It has now been observed that this tendency of high-metals crudes and resids to undergo coking during visbreaking is related to the presence of transition metal values therein, notably nickel and/or vanadium. Such metals can be removed under hydrodesulfurization conditions with frequent catalyst replacement, or in a specially designed cracking unit. However, it is desired to effect more direct treatment, advantageously in a visbreaking operation.